This invention relates to a method of producing a large-sized body of a ceramic honeycomb structure by integration of small-sized honeycomb structure blocks. For example, the large-sized body may be a heat regenerator of a rotary regenerative heat exchanger for either a gas turbine engine or a Stirling engine or a catalyst carrier for an exhaust gas treatment apparatus either in an automobile or in a factory.
A honeycomb structure of a ceramic material is of use, for example, as a regenerator matrix in air preheaters or heat exchangers. Conventionally, a large-sized body of a ceramic honeycomb structure is produced by first producing a plurality of small-sized blocks of the honeycomb structure through firing of molded blocks and then integrating the small blocks into a large-sized body as intended by bonding the small blocks to one another with a ceramic cement of which principal component is a virteous composition. This method is disclosed in Japanese Examined Patent Application Publication No. 39(1964)-10634. However, the product of this method has a shortcoming that it is rather poor in resistance to thermal shocks. In an automotive gas turbine engine by way of example, a honeycomb regenerator body is subjected to exhaust gas temperatures as high as about 1000.degree. C. and severe thermal shocks at the start and stop of the engine. When the regenerator body is a ceramic body made up of small blocks adhered to one another, this body is liable to break in the bonded regions by the influence of thermal shocks. From such a viewpoint, large-sized ceramic honeycomb structure bodies produced by conventional methods are not yet practicable in combustion engines.
Ceramic honeycomb structures are of use also as catalyst carriers in exhaust gas treatment apparatus. Where such apparatus are large in size as in factories and other large-scale facilities, the catalyst carriers are formed into large-sized bodies by the aforementioned bond-integration method. Also in this field, large-sized ceramic honeycomb structure bodies are frequently damaged or broken by thermal shocks.
In general, resistance to thermal shocks becomes a matter of more significant importance to ceramic articles than to metal articles since usually ceramic articles are used at higher temperatures because of their superior refractoriness and accordingly are subjected to thermal shocks or greater intensity. Among physical properties of the material of a rigid and integral article, heat conductivity and thermal expansion coefficient have the most significant influences on the resistance to thermal shocks of the article. If the heat conductivity were infinitely great, the article could be heated and cooled ideally uniformly, so that there is no possibility of the article breaking by thermal stresses attributable to thermal shocks. In reality, however, the heat conductivity never becomes infinitely great. Therefore, it is a common practice to select a material (in this case a ceramic material) having a thermal expansion coefficient as small as possible with the intention of obtaining a product high in resistance to thermal shocks, considering that the magnitude of a thermal stress appearing in the article as the result of nonuniform heating and/or nonuniform cooling becomes small where the article is made of a material having a sufficiently small coefficient of expansion.
A fundamental problem in the conventional bond-integration method of producing a large-sized ceramic honeycomb body is that there is no practicable cement or adhesive identical in thermal expansion coefficient with the ceramic material of the small honeycomb structure blocks to be bonded together. The use of a ceramic cement containing a vitreous composition inevitably results in the presence of heterogeneous regions (bonded regions) in the large-sized product, so that the product remains unsatisfactory in its resistance to thermal shocks.
As another method of producing a large-sized body of a ceramic honeycomb structure, Japanese Examined Patent Application Publication No. 39(1964)-17393 proposes to fix a plurality of small ceramic honeycomb blocks to a metal frame having lattice-like walls by using a plastic composition of which principal ingredient is a ceramic material as a cement. However, the product of this method will have disadvantages such as undesirably high resistance to a gas flow therethrough due to the use of the metal frame, difficulty in tightly bonding the small ceramic blocks to the metal frame due to a great difference in thermal expansion characteristic between the ceramic and the metal and, by reason of such difficulty, significant wear of the unit blocks and the cement where the product is subjected to mechanical vibrations during use.
Accordingly there is an earnest desire for a novel method of producing large-sized bodies of a ceramic honeycomb structure having improved strength and durability.